The present invention relates to methods and apparatus for use with optical fibres, and in particular to methods and apparatus that may be used for optical fibre mode scrambling.
As is known in the art, optical fibres for carrying electromagnetic radiation (such as laser light) can principally have two forms, so-called “single mode” and “multimode” fibres. A “mode” can be thought of as a small range of angles within an optical fibre core that a beam of electromagnetic radiation can propagate within and as, in effect, defining a transmission path of the beam through the optical fibre.
A “single mode” fibre is an optical fibre that only supports one mode (path) through the fibre, for example because its core and numerical aperture are so small that, in effect, radiation can only propagate along the axis of the fibre.
A “multimode” fibre on the other hand will support multiple modes or transmission paths through the fibre, such as a mode that travels along the axis of the fibre and other modes where the beam path travels at an angle to the axis of the fibre and undergoes total internal reflection at the boundary between the core and cladding of the fibre as it propagates down the fibre. Multimode fibres typically have a larger core diameter and/or numerical aperture than single mode fibres, so that they will support multiple transmission modes.
Single mode fibres can be useful because as there is only a single transmission mode (path) through the fibre, the geometry of their output beams (e.g. in terms of the beam diameter and numerical aperture) tends to be stable over time.
However, there can be situations in which a multimode fibre may be preferred. For example, in certain applications, such as fluorescence imaging of samples and flow cytometry, it can be important to achieve a uniform intensity distribution across the sample being analysed (i.e. to have the same intensity across the entire cross-section of the sample-illuminating laser beam). The output beam from a single mode fibre has a Gaussian power distribution, and therefore, to achieve a uniform intensity distribution across the output beam cross-section, additional optics are required.
On the other hand, a multimode fibre more naturally produces a uniform intensity cross-section output beam, and, indeed, would produce an output beam having this form if all the available modes in the fibre were “excited” (i.e. had electromagnetic radiation propagating along them).
A multimode fibre that has all its modes “excited” is typically referred to as being “filled”, since the electromagnetic radiation is propagating in all available modes in the fibre.
However, a difficulty with using multimode fibres is that it can be difficult in practice to “fill” the fibre efficiently with electromagnetic radiation, i.e. to excite efficiently all the transmission modes that the fibre supports. This is because a typical laser beam that is launched into an optical fibre may be a single mode laser (i.e. where the output beam from the laser source propagates in a single, very small range of angles) and/or may have a restricted angular and/or spatial distribution, such that the input beam will not fill the entire acceptance angle cone and/or radiation-accepting cross-section of the multimode fibre. In this case, the input beam may only travel along a few of, rather than all of, the modes that are supported by the fibre (i.e. such that only a subset of the available transmission modes in the fibre will be excited by the laser beam).
Similar issues arise in the context of other electromagnetic radiation sources, such as other more spatially coherent sources, such as super-luminescent LEDs. Again, a super-luminescent LED may be unable in itself to “fill” all the modes of a multimode optical fibre.
A problem that arises when electromagnetic radiation propagates via only some but not all of the modes available in a multimode fibre is that this can make the output beam less consistent and not uniform over time. For example, any disturbance of the fibre in use, or variation in the launch conditions of the beam into the fibre (such as the launch alignment), can cause the “subset” of modes that the radiation is propagating via in the fibre to change. This in turn can lead to a variation in the output of the fibre, for example in terms of the output beam's direction, size, shape, and/or intensity. For example if the input beam is “swept” across the end of the fibre, the beam in the fibre will “sweep” through the available transmission modes in the fibre, and the output beam will correspondingly sweep across a range of output angles.
These effects, together with effects such as relative dispersion and interference between different modes in a multimodal fibre, result in what is commonly referred to as “modal noise”.
One important effect and drawback of modal noise is that the intensity distribution of a beam output by a multimode optical fibre typically is not uniform across the beam's cross-section. This is shown in FIG. 1, which shows an exemplary output beam profile (cross-section) from a typical multimode optical fibre (i.e. in its natural, “raw” state without any adjustment or modification of the fibre or the optical system).
It is accordingly known to try to remove or reduce these effects of modal noise in the output beam from a multimode fibre. Typically this is done by trying to ensure that more, and ideally all, of the modes supported by the fibre are excited (i.e. traveled along) by the beam of electromagnetic radiation, since if all the modes in the fibre are excited (i.e. the beam “fills” the fibre), any variation in, e.g., launch conditions, will not cause a change in the modes propagating in the fibre (as all modes are excited in any event) (and thus should not cause a change in the output beam).
One way to try to achieve this is to try to “launch” the beam from the source into the fibre in such a way that the beam, in effect, “overfills” the core diameter and numerical aperture of the fibre as it enters the fibre. This is intended to ensure that the beam “fills” all the modes in the fibre. Such arrangements typically seek to expand or diffuse the beam before it enters the fibre, i.e. to, in effect, provide an “extended” source that can then completely “fill” the fibre. Another known similar approach is to attempt to raster scan the beam across the input face of the fibre core, again so as to try to ensure that the beam “fills” the fibre.
However, a drawback with these arrangements is that the necessary optical, etc., arrangements tend to result in loss of power from the beam. The arrangements can also be complex to use and maintain.
Another known technique for trying to excite all modes in a multimode optical fibre is known as “mode scrambling” (and the devices used to achieve this are typically referred to as “mode scramblers”). FIG. 2 shows schematically a perfectly “scrambled” output beam profile from a multimode fibre. It can be seen that the intensity of the beam is substantially uniform across the cross-section of the beam. The aim of mode-scrambling is to, in effect, make the overall geometry of the output beam (and in particular at least the beam diameter and numerical aperture) more stable over time, and, e.g., more robust to variable launch conditions (such as launch alignment). Mode-scrambling typically involves introducing perturbations into the optical fibre along its length, which perturbations will act to “excite” different and/or more modes in the fibre.
One known mode scrambling technique involves placing a series of bends of differing bend radii along the length of the optical fibre, for example by placing the fibre between two irregularly corrugated surfaces. This serves to “excite” extra propagation modes in the fibre, because at each bend, the beam will strike the core/cladding interface at a different angle or angles, thereby changing the angular distribution of modes in the fibre. The intention is that by forcing the beam around the bends of different radii as it travels along the fibre, these internal reflections will act to sweep the beam through and into all the modes that the fibre supports (i.e. to cause the beam to “fill” the fibre).
However, a disadvantage of this arrangement is that transmission losses may occur at each bend in the fibre, because, for example, one or more of the propagation modes may strike the cladding at an angle that does not cause total internal reflection (such that radiation is lost into the cladding).
It is also known to try to achieve mode scrambling by using a “daisy chain” of multimode fibres of different core diameters and/or numerical apertures. In this case, the intention is that as the beam passes from one fibre to the next in the “daisy chain”, different or more propagation modes will be excited.
However, this arrangement can again cause transmission losses as the beam propagates through the fibres, as there may, for example, be losses at each interface between different fibre sections.
A third known mode scrambling technique is to include “scattering centres”, such as non-optically clear regions, in a multimode optical fibre. Again, the intention is that the electromagnetic radiation (e.g. laser) beam will be “scattered” into different or more modes of propagation by the scattering centres. However, there is again a risk of transmission losses at each scattering centre that the beam encounters.
It is also known to use combinations of two or more of these techniques, but again this does not avoid the problem of transmission losses being caused by the “mode scrambling”, etc.
Therefore, there remains scope for improvement to current mode scrambling techniques for use with optical fibres.